Process for obtaining nitrogen tetroxide



.Sgpt. 28, 1948. a. w. BATCHELDER ET AL PROCESS FOR OBTAINING NITROGEN TETROXIDE Filed Feb. 13, 1943 ATTORNEY Patented Sept. 28 1948 PROCESS FOR OBTAINING NITROGEN I TETROXIDE George Washington bert W. Hawkins,

Batchelder, Mantua, and Al- Woodbury, N. J assignors to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware 1 Application February 13, 1943, Serial No. 475,724

11 Claims. 1

This invention is directed to a process and apparatus for the manufacture of nitrogen tetroxide (N02) and in particular to the production of N02 from the oxides of nitrogen or from one or more substances containing at least one of said oxides.

Nitrogen tetroxide is of commercial importance because of its properties and various uses. It serves well as an oxidizing, nitrating, bleaching and diazotizing agent and is often used as an intermediate product.

Several processes have been advanced for the production of N02 but they have not fully solved the problems involved. The primary drawback has been the high cost of production and the impurity of the final material, or either of these. Method-s of the prior art for producing N02 include treating gaseous mixtures of nitrogen and oxygen in electric furnaces, burning carbon monoxide under pressure in the presence of oxygen and nitrogen, exploding a compressed mix ture of oxygen and nitrogen, subjecting a mixture of air and NO to vigorous agitation at a low temperature,passing a mixture of hydrocyanic acid and air at 600 C. in contact with a catalyst, decomposing nitric acid by heating and rapidly cooling the mixture of evolved gases, and other procedures.

An object of the present invention is to provide a new and improved process and apparatus for manufacturing nitrogen tetroxide. Another object'is aprocess for producing substantially pure N02. A further object is a process for producing N02 comprising oxidizing ammonia with oxygen, air or other oxygen-bearing gases in the presence of a heated catalyst, concentrating the N02 by means of sulfuric acid and recovering the N02 therefrom. An additional object is such a process characterized by its economy and emciency. These and still further objects and advantages of our invention will be apparent upon a perusal of this specification.

In carrying out our invention, in general, oxides of nitrogen obtained from the ammonia oxidation or other suitable process are absorbed in an excess of concentrated sulfuric acid, resulting in a mixture comprising essentially nitrosyl sulphuric acid, to which dilute nitric acid is added. The resulting mixture is preheated to between about 85 and 110 C. and introduced into a. stripping tower from which N02 is evolved and condensed. The residual sulfuric acid is exited from the base of said tower. A more detailed disclosure of our invention is made hereinbelow. According to the preferred embodiment of the present invention, about 70 to 95% strength sul-.

furic acid is introduced into the upper region of an absorption tower countercurrent to approx imately an equimolar mixture of N0 and N02 entering the lower portion of the tower. How-' ever, the concentration of the sulfuric acid may be varied widely depending upon the use to be made of the N02. For example, if it is desired to use the N02 for certain purposes, as low as to sulfuric may be employed. The mixture obtained thereby is passed into a preheater towhich nitric acid'of about 30 to 60% strength is added. The resulting mixture, maintained between approximately and 110 0., is introduced into a stripping or denitrating tower countercurrent to steam conveyed into said tower near the base thereof. The temperature of the stripping tower is maintained between about and 115 C. In addition to nitric acid and water vapors, N0 and N02 are evolved. Nitric acid vapors pass out the top of said tower into a preliminary condenser, and the condensate refiuxes into the tower countercurrent to N0 and oxidizes it to N02. The denitrated or residual sulfuric acid is withdrawn from the bottom of the stripping tower. Nitrogen tetroxide passes through the preliminary condenser into a total condenser and is liquefied. Up to 99% or higher N02 is obtained by this process.

In all instances herein where a tower is referred to, it is to be construed as being adequately provided with packing material or fractionating plates. These towers may be of any type absorption, stripping, denitrating or distilling tower or other satisfactory apparatus which will accomplish the desired result, and may be constructed either of acid-resistant iron, chemicalware or any like material which will serve the purpose.

In order to disclose our invention further and to point out the particulars thereof, reference is made to the'accompanying diagrammatic representation which, however, is to be regarded only as a desirable way of carrying out said invention and is not to be taken as a limitation thereon. Referring to the drawing, sulfuric acid of from '70 to, strength is introduced through line I into an absorption tower 2, the rate of flow being controlled by valve 3. This acid flows down the tower countercurrent to approximately an equimolar mixture of N0 and N02 entering through conduit 4, the rate of passage being regulated by valve 5. Said tower is vented at 6 to allow the escape of waste gases. Preferably the temperature of the absorbent (sulfuric acid) is maintained between 20 and 30 0., since a departure from this temperature range tends to lowerthe absorption. i The resulting liquid, containing a.

mixture of nitric and nitrosyl sulfuric acid in sulfuric acid, is passed from the bottom of the absorption tower through line 1' into the base of a preheater 8. Although this preheater, as shown, is steam-jacketed other heating means may be used. The preheater may be packed a portion of its length to serve also as a mixer. Nitric acid having a strength from 30 to 60% is introduced from storage tank 2| into the bottom of the preheater through line 9, the rate of flow being regulated by valve It. The rate of flow desired depends on the amount required to maintain approximately an equimolar ratio of nitric to nitrosyl sulfuric acid. The mixture of acids is preheated to between about 85 and 100 C. and passed through line I! into a stripping or denitrating tower I2, the rate of flow being controlled by valve 13. This mixture flows down said tower countercurrent to steam introduced through line H! at the bottom thereof, the rate of flow of the latter being regulated by means of valve IS. The temperature of the denitrating tower is maintained between about 90 and 115 C. Denitrated sulfuric acid is exited through line l6 at the bottom of the tower. N0, N02, nitric acid vapors and water vapors evolve from the mixture and pass out of the tower through conduit [1. Nitric acid vapors liquefy in the preliminary condenser l8 and reflux into the tower countercurrent to NO and oxidize it to N02. N02 vapors pass through condenser I8 and into a total condenser IS in which they are liquefied. Pumps 22 and 23 serve to convey the materials through the system. Liquid N02 up to 99% purity or better Usually some nitric acid is formed in reaction (1) since N02 and sulfuric acid react to form nitric and nitrosyl sulfuric acid, From this equation it can be seen Why an equimolar ratio of NO to N02 is preferred. The resulting mixture is butted with nitric acid to produce approximately an equimolar ratio of nitric to nitrosyl sulfuric acid.

The following equation indicates a reaction which occurs in the denitrating or stripping tower.

Likewise from Equation 2 it may be seen why an equimolar ratio of nitric to nitrosyl sulfuric acid is preferred. There are several, although not readily apparent, advantages of using nitric acid in our process. It is known that nitrosyl sulfuric acid is extremely stable in the presence of concentrated sulfuric acid. Such a mixture may be heated to its boiling point Without any considerable decomposition of the nitrosyl sulfuric acid. However, we have found that the decomposition of nitrosyl sulfuric acid in sulfuric acid is great y facilitated by the presence of 30 to 60% strength nitric acid, the ratio of nitric to nitrosyl sulfuric being approximately one. Other than diluting the absorbent, nitric acid itself supplies additional N02.

Lower oxides of nitrogen, for example NO or N203, may form during the deni-tration according to the following equations.

However. these are immediately oxidized .to. N02 by the refluxing nitric acid. This acid passes back into the mixture in the tower and aids in further decomposing nitrosyl sulfuric acid.

From the foregoing particularistic description of our invention, variations may be suggested to those skilled in the art; however, these and many other modifications are included within the language of the appended claims. That is, changes may be made in the strengths and relative proportions of compositions, in the conditions of reactions, and in the form, construction and arrangement of the parts herein without departing from the scope of the invention and without sacrificing its advantages. Although the drawing pertains to a continuous process. the invention may be practiced by processing batches. Thus the preheater is not essential, since nitric acid may be mixed with the material from the absorption tower and th resulting mixture brought to the desired temperature after introducing the same into the stripping tower. Detailed disclosure and a discussion of the theories or principles, including reactions, underlying our invention are for the purpose of clarity only and are not to be construed as a definition of. the scope of said invention, reference being had for this purpose to the following claims.

We claim.

1. A process for obtaining nitrogen tetroxide in relatively pure form which comprises absorbing nitrogen oxides in strong sulfuric acid, introducing nitric acid into the resulting mixture, distilling the mixture, condensing and refluxing the nitric acid and water vapors, causing said nitric acid to react with and oxidize the N0 present to N02 and simultaneously itself to become reduced to N02, evolving and subsequentl condensing the nitrogen tetroxide. and separating it as a liquefied product.

2. In the method of obtaining nitrogen tetroxide in relatively pure form the steps which comprise absorbing nitrogen oxides in sulfuric acid of between approximately 70 and strength, introducing nitric acid of between approximately 30 and 60% strength into the mixture so obtained at a temperature between about 90. and 0., distilling the mixture, condensing and refluxing the nitric acid and water vapors, causing said nitric acid to react with and oxidize the NO present to nitrogen tetroxide While simultaneously becoming reduced itself to nitrogen tetroxide, evolving and subsequently condensing the nitrogen tetroxide, and separating it as a liquefied product.

3. A process for obtaining nitrogen tetroxide in relatively pure form which comprises introducing nitrogen oxide bearing gases into an absorption tower countercurrent to sulfuric acid, passing nitric acid into the resulting mixture, introducing the mixture thus formed into a denitrating tower passing a heating fluid countercurrent thereto in direst heat exchange therewith, evolving nitrogen oxides, nitric acid and water vapors, condensing and refluxing nitric acid and Water vapors into said tower, causing said nitric acid to react with and oxidize the NO present to nitrogen tetroxide while simultaneously becoming reduced itself to nitrogen tetroxide. passing the evolved nitrogen tetroxide into a condenser, and li-quefying and separating the same.

4. A process for obtaining nitrogen tetroxide in relatively pure form which comprises introducing nitrogen oxid bearing gases into an absorption tower countercurrent to sulfuric acid, introducing the resulting mixture into a preheater, passing nitric acid into said prehea'ter, introducing the mixture thus formed into a denitrating tower passing a heating fluid countercurrent thereto in direct heat exchang therewith, evolving nitrogen oxides, nitric acid and water vapors, condensing and refluxing nitric acid and water vapors into said denitrating tower, causing said nitric acid to react with and oxide the NO present to nitrogen tetroxide while simultaneously becoming reduced itself to nitrogen tetroxide, passing nitrogen tetroxide into a condenser, and liquefying and separating the same.

5. In the process of obtaining nitrogen tetroxide in relatively pure form, the method of concentrating said compound which comprises bringing nitrogen oxides into contact with strong sulfuric acid, introducing the resulting mixture into a preheater, passing relatively weak nitric acid into said preheater, introducing the mixture obtained into a denitrating tower passing steam countercurrent thereto and in direct heat exchange therewith, condensing and refluxing the evolved nitric acid and water vapors into said tower causing said nitric acid to react with and oxidize the NO present to nitrogen tetroxide while simultaneously becoming reduced itself to nitrogen tetroxide, passing the evolved gaseous nitrogen tetroxide into a condenser, and liquefying and separating the same.

6. The method for obtaining nitrogen tetroxide in relatively pure form which includes absorbing nitrogen oxides in between about 70 and 95% strength sulfuric acid, introducing between about 30 and 60% strength nitric acid into the resulting mixture, passing the mixture thus obtained into a denitrating tower passing a heating fluid countercurrent thereto and in direct heat exchange therewith, evolving nitrogen oxides, nitric acid and Water vapors, condensing and refluxing nitric acid and water vapors into said tower, causing said nitric acid to react with and oxidize the NO present to nitrogen tetroxide while simultaneously becoming reduced itself to nitrogen tetroxide, passing the nitrogen tetroxide into a condenser, and liqueiying and separating the same.

7. A process of obtaining nitrogen tetroxide in relatively pure form which comprises absorbing nitrogen oxides in sulfuric acid, maintaining said acid at a temperature between about and 30 C., introducing the resulting mixture into a preheater, maintaining said preheater at a temperature between about 85 and 110 C., passing nitric acid into said preheater, introducing the mixture thus obtained into a denitrating tower passing steam countercurrent to the mixture and in direct heat exchangetherewith, maintaining the temperature of said tower between about 90 and 115 0., condensing and refluxing the evolved nitric acid and water vapors into said tower causing said nitric acid to react with and oxidize the NO present to nitrogen tetroxide while simultaneously becoming reduced itself to nitrogen tetroxide. condensing the evolved nitrogen tetroxide, and liquefying and separating the same.

8. The method for obtaining nitrogen tetroxide in relatively pure form which comprises bringing nitrogen oxides into contact with sulfuric acid of between approximately 70 and 95% strength, introducing the resulting mixture into a preheater, passing nitric acid of between approximately 30 and 60% strength into said preheater, introducing the mixture thereby formed into a denitrating tower passing steam countercurrent to the aforesaid mixture and in direct heat exchange therewith, condensing and refluxing the evolved nitric acid and water vapors into said tower causing said nitric acid to react with and oxidize the NO present to nitrogen tetroxide while simultaneously becoming reduced itself to nitrogen tetroxide, subsequently condensing the evolved nitrogen tetroxide and liquefying and separating the same.

9. A process for obtaining nitrogen tetroxide in relatively pure form including absorbing a mixture of at least two nitrogen oxides in sulfuric acid of between approximately 70 and 95% strength, maintaining said acid at a temperature between about 20 and 30 C., introducing the resulting mixture into a preheater, maintaining said preheater at a temperature between about and 110 C., passing nitric acid of between approximately 30 and 60% strength into said preheater, introducing the mixture thus obtained into a denitrating tower passing steam countercurrent thereto and in direct heat exchange therewith, maintaining the temperature of said tower between about and 115 C., condensing and refluxing the evolved nitric acid and water vapors into said tower causing said nitric acid to react with and oxidize to nitrogen tetroxide the lower nitrogen oxide while simultaneously becoming reduced itself to nitrogen tetroxide, liquefying and recovering th liberated nitrogen tetroxide.

10. The process of claim 9 in which the mixture of nitrogen oxides comprises NO and N02.

11. A continuous process for obtaining substantially pure nitrogen tetroxide which comprises introducing a substantially equimolar mixture of NO and N02 into an absorption tower counter-current to sulfuric acid Of between approximately 70 and strength maintained at a temperature between about 20 and 30 0., passing the resulting mixture into a steam-jacketed preheater maintained at a temperature between about 85 and C., introducing a sufiicient amount of nitric acid of between approximately 30 and 60% strength into said preheater to maintain a molar ratio of nitric to nitrosyl sulfuric acid of substantially one, passing the mixture thereby obtained into a denitrating tower passing steam countercurrent thereto and in direct heat exchange therewith, While maintaining the temperature of said tower between about 90 and C. whereby a gaseous mixture of nitric acid vapors, water vapor, NO and N02 is evolved, condensing and refluxing the evolved nitric acid and water vapors into said tower countercurrent to the evolved NO whereby said NO is oxidized to NO: and subsequently liquefying and separating nitrogen tetroxide.

GEORGE WASHINGTON BATCI-IELDER. ALBERT W. HAWKINS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,197,295 Jensen Sept. 5, 1916 1,236,662 Berkeland Aug. 14, 1917 1,324,255 Jensen Dec. 9, 1919 1,590,043 Lentz June 22, 1926 1,740,549 McKee Dec. 24, 1929 1,755,768 Busching Apr. 22, 1930 1,989,267 Caro Jan. 29, 1935 2,169,826 Wendlandt Aug. 15, 1939 FOREIGN PATENTS Number Country Date 237,937 Great Britain Aug. 11, 1925 292,951 Great Britain Oct. 15, 1929 Certificate of Correction Patent No. 2,450,105. September 28,1943;

GEORGE WASHINGTON BATOI-IELDER ET AL. It is-hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 4, line 60, claim 3, for direst read direct; column 5, line 6, claim 4, for the word oxide read oxidize; line 17, claim 5, after mixture insert thereby;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 21st day of December, A. D. 1948.

THOMAS F. MURPHY,

Assistant Gammissimter of Patents. 

